when do you fly each routing

Assuming your question applies to Sim, fly any routing that pleases you. It's not only a game but a pretty good teaching tool. Using all the navigation options will make you a more well-rounded sim pilot.

In the real world, direct routings are the option of choice whenever they are available for reasons of economy. It almost always requires less fuel and time to fly direct than to fly any non-direct route.

Low altitude routes are used by light aircraft that cannot reach flight levels. In some cases, aircraft that are capable of high altitude flight also use low altitude airways but usually only for very short flights. High altitude airways are used when possible because it is more economical to do so and it also takes you above most weather, thereby increasing passenger comfort and safety of flight.

well...i think i have it down...but in the flight planner, I selected a route from Heathrow to DFW, and decided to see what it looked like when i chose high altitude airways...i pratically fly around the world, and same with low altitude airways..When I try to select VOR to VOR, it doesn't allow me to..some sort of error comes up, which doesn't look like it's a mistake...i also heard that if I flly direct GPS, it's not realistic, In the real world, if a Lufthansa and a KLM depart at the same time, flying the same route, they only have a few miles between them, because Lufthansa flies faster aircraft than KLM, if you know what I mean..in that sort of example, what would you do?

When using airways across long distances it is quite a common problem for it to give you a very strange and long route. In this case flying direct gps would probably be more realistic than the airways. Alternatively you can delete some of the waypoints on your flight route to make it more direct. With regards to your other question, I have taken off just after AI aircraft and raced them to their destination. I have never had a problem but I'm not sure what route the AI aircraft takes so I'm not sure. The aircraft for each airline may also be flying at different altitudes. Not quite sure. Just an educated guess.
😀 ANdy

The old system had 2,000 foot altitude seperation between aircraft. With the new Reduced Seperation Minima it goes to 1,000 feet seperation if the aircraft has the propoer equipment to do so.

The airways are becoming too crowded and the new system is in place to relieve the structure.